1. Field of the Invention
The invention relates to a transmission system comprising a transmitter for transmitting binary data signals via a transmission means of a high-pass nature and a receiver, the transmitter comprising a data source for generating data bits in synchronous consecutive bit intervals having a duration T and a code converter for converting each data bit into a corresponding data symbol, and the receiver comprising a receive filter and a data detector for detecting the transmitted data symbols.
2. Background of the Invention
A similar system is known from U.S. Pat. No. 3,846,583. More specifically, the specification of this patent describes a code for converting data bits into data symbols. The code is designated WAL2 code or Top Hat code. The code results in an improved reduction of low-frequency components in the spectrum of the data symbols, over the biphase code. The data symbols obtained by means of this WAL2 code have a duration T and a form corresponding to a cycle of a square-wave carrier, whose frequency equals the bit frequency of the data signal and whose phase has been shifted through 90.degree. relative to the phase of the bit clock of the data signal.
This WAL2 code can be used for the transmission of binary data signals in mixed systems known as DAV systems (Data Above Voice). In such system, analog information in the form of a speech signal, with relevant additional information such as metering pulses, is transmitted and the frequency range above the band of this analog information is used for transmitting a suitable decoded data signal. However, when applying the WAL2 code, the transmission of the analog information can be disturbed by the low-frequency components of the encoded data signal. The use of high-pass filters of a relatively high cutoff frequency to reduce the disturbing effect of these low-frequency components produces considerable intersymbol interference in the encoded data signal. The intersymbol interference extends over a relatively large number of bit intervals and necessitates complicated adaptive equalisers in the receiver to significantly reduce the disturbing effect on the detection of the data symbols.